Combined oscillator-detector circuits



March 26, 1940.

COMBINED OSCILLATOR-DETECTOR CIRCUiTS Original Filed April 27, 1933 2 Sheets-Sheet 1 no +2501! BIAS INVENTOR CHARLES TRAVIS c. TRAVIS 2,194,512

ATTORN EY Patented Mar. 26, 1940 2,194,512,

COMBINED OSCILLATOR-DETECTOR CIRCUITS Charles Travis, Philadelphia, Pa, assignor to Radio Corporation of America, a corporation of Delaware Original application April'27, 1933, Serial No.

668,231. Divided and this application Septembet 3, 1937, Serial No. 162,274

9 Claims. (Cl. 250-20) An object of the present invention is to provide application Serial No. 642,544, filed Nov. 14, 1932,

a circuit arrangement whereby a vacuum tube Patent No.2,074,014, March 16, 1937, in the names oscillator havingaplurality of grids will generate of 'K. Chittick and W. L. Carlson. Lead oscillations of: substantially constant frequency. 20 may be connected to any other source of auto- The present application is a division of my apmatic volume control bias, as well known in the 5 plication, Serial No. 668,231, filed April 27, 1933, art, to control the gain of the tube, Radio fre- Patent No. 2,153,778, April 11, 1939. Y quency energy of any desired frequency, such as A further object is to provide such a tube 500 to 1500 kilocycles, may be applied. to the input oscillator in which the oscillation strength or terminals 19 of primary 2|. As shown, the local voltage generated may be maintained suboscillator circuit includes 'coil II and variable stantially constant. condenser 12 by which the oscillation frequency A still further object is to provide a combined may be adjusted to produce any desired beat oscillator detector, which may for example be the frequency. This circuit may be connected in a first detector of a superheterodyne receiver, in known manner to the oscillator control grid I 35 which there is substantial freedom from frequency through a condenser ID, a suitable grid leak redrift. sistor 9 being provided and connected to grid l Another object of the invention is to further and the cathode C. The normal steady bias of neutralize the capacity existing between the plate the grid 4 may be provided in known manner by and a grid of a vacuum tube having a plurality a resistor 'l'connected between cathode C and of grids, including a screen grid. Still other obground, resistor I being shunted by a condenser 8. 20

jects of the invention will be apparent to those The plate 6 may be connected to a constant skilled in the art as the description thereof voltage source of abou 250 Vo t owa proceeds. parallel circuit comprising coil I6 and condenser Circuit arrangements showing embodiments of H, the latter of which may be adjustable to tune the invention are illustrated in the accompanying he p ate Circuit t0 t e desired beat q y, drawings in which: which may be, for example, 175 kilocycles. The Figs. 1 and 2 show a vacuum tube having a oscillator'anode electrode 2'is shown connected to plurality of grids arranged as a combined oscilthe 250 volt source through a resistor l4, shunted lator and detector; by a condenser l5, and a tickler coil I3 which is Figs. 3 and 4 show circuits embodying my ininductively coupled to coil ll. This coupling be- 30 vention in connection with a combined oscillator tween coils l3 and l l provides the'necessary feeddetector having five grids; and 7 back of energy to the grid circuit l ll2, to cause Fig. 5 shows a circuit similar to that of Fig. 1 the tube to generate oscillations. The screen with a neutralizing arrangement according to my grids 3 and 5 may be connected together coninvention. ductively either within, or without, the tube, and 35 This invention relates in general to circuit armay have a steady positive voltage of 50 to 100 rangements useful in connection with a Vacuum volts applied thereto from any desired voltage tube having several grids as disclosed in the ap- Source (not Shown) y 3 lead e T can plication filed in the name of J. C. Smith on Jan. be of the well-known 2A7 type.

40 31, 1933, Serial No. 654,421 and is in certain re- Now, I have found that, when the feedback spects, an improvement on the circuit arrangecircuit shown in Fig. 1 is used, as the signal conments disclosed therein. trol grid 4 is biased back more negatively by the Referring to Fig. 1 a vacuum tube T is shown AVC circuit, the local oscillation strength inhavlng a heater H which heats the electron emitcreases, that is, the magnitude of the locally proting cathode C, and a plate 6. Interposed, or duced oscillations increases. I also found that 45 located, in the electron stream between the the oscillation frequency changes. In one incathode and plate are a series of grids, I, 2, 3, l stance I foundthat with a typical oscillator coil and 5 arranged in the order shown, grid 1 being tuned by condenser E2 to 1,000 kilocycles, the frenearest the cathode and grid 5 being nearest quency decreased 500 cycles per second when the the plate 6. A tunable input circuit, consisting bias on grid 4 was changed from -3 volts to cut- 50 of secondary 22 and variable condenser i8, is conon (about -30 to 35 volts) nected to the signal control grid 4, the lead 20 The circuit arrangement shown in Fig. 2 is connected to the opposite side of coil 22 being essentially the same as that of Fig. 1, except that connected to the output circuit of the second dein this case the tickler coil i3 is also included in tector of the receiver as shown, for example, in the circuit of plate 6 so thatthe platecurrent of the tube flows through it, in addition to the current of oscillator anode electrode 2. With this arrangement I found that the oscillation strength, or magnitude, decreases as the control grid 4 is given a more negative bias. I also found that at an initial oscillator'frequency of 1,000 kilocycles, the frequency increased by 700 cycles as the bias on grid 4 was changed from 3 to about volts.

In order'to secure substantially constant oscillation strength and frequency as a variable bias is applied to control grid 4, I take a portion only of the plate feedback, and combine it with the feedback of the oscillator anode electrode 2. This may be accomplished, as shown in Fig. 3, by tapping the feedback coil l3 at such a point that the two opposing effects above mentioned balance out. With a 2A7 type tube I found that the turns in the lower tapped portion l3 were one third of the total number of turns in the entire tickler coil I3.

By using two separate tickler coils, each inductively coupled to coil H as shown in Fig. 4, I was able to secure a similar result. By making the tickler coil 28, in the plate circuit, of one third the number of turns of tickler coil I3, it was found that the oscillation strength and frequency remained substantially constant, as the bias on signal grid 4 was changed over a substantial voltage range. In the particular tube used in these tests, the oscillator control grid I was of the sharp cut-off type wherebyless voltage was required to produce the desired oscillation strength, whereas, grid 4 was constructed to give remote cut-off or producea result similar to that of a variable mu tube.

In Figs. 3 and 4 I have shown the intermediate frequency tuned circuit I 6, ll inductively coupled to a secondary 24 tuned to the intermediate frequency by a condenser 25. This circuit maybe connected as the input circuit of an intermediate frequency amplifier 21, being connected to the grid 26 thereof. For broadcast reception circuits Hi, I! and 24, 25 are preferably tuned to 175 kilocycles, or any other desired beat frequency. To secure this frequency condenser I2 is initially adjusted so that the oscillation frequency is higher, or lower, than the incoming signal frequency by an amount equal to the desired beat frequency.

As applied to a superheterodyne receiver, therefore, Iam enabled by this invention to prevent detuning of the receiver due to change, or drift, in the oscillator frequency caused by change in the grid bias of the signal control grid 4. Also,

it will be understood that it is equally useful where the bias of grid 4 is changed by a manual volume control means in lieu of the AVG means shown.

A further aspect of the invention is disclosed in Fig. 5 where the circuit shown is generally similar to that of Fig. 1. A neutralizing circuit has been added, as shown, comprising CN and a coil 29 inductively coupled to coil 22. Coil 29 may consist of a few reversed turns wound around the low potential end of the radio frequency secondary coil 22. Even in the presence of the screen grids 3, 5 in a tube of thetype above described, it is found that a capacity of about one quarter micro-microfarad (0.25) exists between plate 6 and grid 4, and it is the purpose of the neutralizing circuit shown to neutralize this ca pacity. At the radio frequency signal voltage applied to grid 4, the load in the plate circuit is essentially capacitive. The effective plate circuit capacity is always less than the physical capacity I! used to tune coil It to the intermediate. beat frequency. Calling f1=intermediate frequencyy f2=the applied radio signal frequency; C= va1ue of condenser l1 and C =e1fective capacity in the plate circuit Grid conductance Gmx equivalent to 250,000'ohms. The effect ofthis is =4 micromhos substantially equivalent toconnecting a resistor of this value across the coil 22 of the tuned signal circuit, and it may seriously affect both the gain and selectivity of the receiver; or the effective plate circuit capacity Cp may be-much smaller, as would be the case if the intermediate frequency were higher than 465 kilocycles, thus ap preaching more nearly the lowerfrequency end of the broadcast band, say 525 kilocycles. Further, as the capacity to be neutralized is very small, and its effect is degenerative and not regenerative, a neutralizing condenser of fixed value may be used; that is, the circuit is not one requiring individual adjustment of the neutral izing condenser for each individual receiver.

While the neutralizing circuit described has been shown as applied to a circuit as shown-in Fig. 1 is will be understood that the same circuit may be employed in connection with the circuits in Figs. 3 and 4, the coil 29 being inductively coupled to coil 22 and the neutralizing condenser Cu being connected to plate circuit 6 in the manner described.

In the circuits shown in Figs. 3 and 4, I found the following values to give satisfactory results I when the source of voltage for plate 6 was 250 volts, voltage applied to screen grid 3, 5:50 to volts, voltage applied to anode electrode 2:100

volts. This latter voltage may be easily secured,

as shown, by using a resistor l4 having a value of approximately 20,000 ohms.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of myinvention, as set forth in the appended claims.

What I claim is: v

1. In combination, a combined oscillator and detector comprising a vacuum tube having an oscillator grid, anode electrode a signal control grid, a screen grid mounted on each side of said signal control grid, a cathode and a plate, a circuit tunable to the incoming signal frequency connected to said signal control grid, a tunable oscillator circuit connected between said oscillator grid and cathode, a circuit connected to said anode electrode and including a feedback con, nection to said oscillator circuit to cause said tube to generate oscillations of a frequency higher than the incoming signal frequency by a desired beat frequency, means for applying a positive voltage to said screen grid, a plate circuit including a circuit tuned to said beat frequency and means to prevent reaction of said last named circuit on said circuit tuned to the signal frequency due to thecapacity coupling between said plate and said signal control grid.

I 2. The combination defined in the preceding claim 1 in which said reaction preventing means comprises a condenser and coil connected in series, said coil being inductively coupled to thesignal tuned circuit and a conductive connection between one side of said condenser and lthe plate of said tube.

3. In combination, an electron discharge tube having at least a cathode and an output electrode, an oscillator grid, an oscillator anode and I a signal grid being disposed in the electron stream betweensaid cathode and output electrode, said signal grid being disposedbetween the oscillator anode and said output electrode, a signal input circuit connected between the cathode and signal grid, 2. network, including a resonant circuit tuned to a predetermined local oscillator frequency, respectively coupling the oscillator anode and oscillator grid, an intermediate frequency circuit connected to said output electrode,'means reactively coupling the output electrode and said local oscillation frequen'cy'resonant circuit, and means for establishing said signalgrid at a predetermined negative direct current potential with respect to the cathode.

4. In combination, an electron discharge tube having at least a cathode and an output electrode, an oscillator grid, an oscillator anode and a signal grid being disposed in the electron stream between said cathode and output electrode, said signal grid being disposed between the oscillator anode and said output electrode, a signal input circuit connected between the cathode and signal 7 grid, a network, including a resonant circuit tuned to a predetermined local oscillator frequency, reactively coupling the oscillator anode and oscillator grid, an intermediate frequency circuit connected to said output electrode, means I local oscillation amplitude.

lator anode electrode and the mixer output electrode, to the oscillator grid electrode, an automatic gain control means for increasing the negative bias on the signal grid of the tube as the received signal amplitude increases and the feedback from each of said electrodes-being pro- 'portioned to maintain substantially constant oscillation amplitude as said negative bias varies.

6. A superheterodyine receiver including a combined local oscillator-first detector network em ploying a tube provided with oscillator and mixer sections, said oscillator section including a grid electrode and an anode electrode and said mixer section including a signal grid and an output electrode, automatic volume control means for varying the bias of the signal grid, and means providing a" radio frequency energy feedback from each of the oscillator anode and the mixer output electrode to the oscillator grid electrode, and said feedbacks being relatively proportioned to render the oscillation strength and frequency substantially independent of the signal grid bias variation.

7.In a receiver as defined in claim 6, said mixe sectionsignal grid being constructed to have a remote cut-off characteristic, and said oscillator section grid has a sharp cut-ofi characteristic.

8'. Inv an oscillator-detector circuit comprising a tube provided with oscillator section electrodes and detector section electrodes, said detector section including a signal input electrode and an output electrode, a source of signals coupled to the signal input electrode of the detector section, a local oscillation circuit coupled to the oscillator electrodes to produce oscillations of a frequency diiiering'from the signal frequency by an interquency energy feedback between the output electrode of the detector section and the said oscillation circuit, and means for varying the direct current voltage of said signal input electrode thereby to control the amplitude of the intermediate frequency energy.

9. In a detector-oscillator system using a'tube provided with oscillator section electrodes and detector section electrodes, said detector section including input electrodes and an output electrode, a source of signals coupled to the input electrodes of the detector section, a local oscillation circuit-coupled to the oscillator electrodes,

a beat frequency output circuit coupled to the mediate frequency, means providing radio fredetector section output electrode, means feeding back radio frequency energy from the said output electrode to said oscillation circuit, and automatic gain control means responsive to an increase in signal amplitude for decreasing the CHARLES TRAVIS. 

